The activated sludge treatment method is a very exceptional waste water treatment method, and is accordingly widely used for waste water treatment. Various treatment systems adapted to different types of waste water have been proposed.
In conventional activated sludge treatment methods, utilizing the natural food chain, various microbes, from soil bacteria to large protozoa, are utilized.
An example of an existing activated sludge treatment method is shown in FIG. 4.
Waste water is introduced into a waste water conditioning tank, and subjected to pre-treatment as needed, eliminating coarse to fine matter dispersed in the waste water, in a pressurized flotation tank. In the case of waste water having a high oil content, separation-flocculation using a flocculant is performed by way of a primary treatment. For example, inorganic flocculants such as aluminum sulfate (known as “aluminum sulfate, anhydrous”) or polyaluminum chloride (known as “PAC”), or organic flocculants such as polyacrylamide based polymers or the like, are added to effluent to break down the emulsified state and separate the oil component and the water component. However, in such cases, a large amount of flocculant, in the form of sludge, is generated. During treatment in the pressurized flotation tank, contaminant substances contained in the waste water are removed through adhesion to the inorganic flocculant, forming floating scum and generating sludge. The sludge is collected in a waste water holding tank.
Soluble organic matter suited to microorganism treatment is fed as waste water for treatment to an aeration tank, and after aeration treatment with activated sludge, the waste water for treatment is separated from the activated sludge and released, while the separated concentrated active sludge is collected in a sludge holding tank, a portion thereof being recirculated to the aeration tank for use as return sludge.
The sludge collected in the sludge holding tank is fed to a dewatering apparatus and transformed into a dewatered cake, which is buried in landfill, or disposed of as bacterial fertilizer or through incineration.
Such activated sludge treatment methods have problems such as the following, which need to be solved.
(1) In the case of waste water containing high levels of proteins, modified proteins, cellulose-starch, fats and oils, and other such persistent substances, large amounts of dewatered cake which emits a foul odor are generated as excess sludge. To reduce the amount of excess sludge, there has been proposed a method of increasing the size of the aeration tanks and digesting for a prolonged aeration time (lagoon system, oxidation ditch). However, with this method, in cases of large loads imposed by copious amounts of waste water, an extremely large aeration tank is required, and therefore problems such as lack of a site or high construction costs may arise. Also, once bulking or a treatment anomaly has occurred, considerable expense and several days may be needed for recovery, and consistency of operations at the plant may be a problem.
(2) When performing microorganism treatments, it is necessary to bring the inflowing waste water to neutral pH. However, when chemicals are added to neutralize, in addition to outlays for the chemicals, accelerated corrosion of equipment can become a problem. Another problem is that the pH of the released treated water varies greatly when the pH of inflowing waste water is regulated.
(3) With activated sludge methods, inflow of toxic substances that could render the activated sludge sterile will damage the activated sludge, which tends to give rise to a loss of waste water purifying function, or to filamentous bulking. However, as it is not possible to completely avoid the presence in waste water of low levels of bactericides or substances having adverse effects on activated sludge, elimination of such harmful substances is a problem.
(4) In activated sludge treatment plants in which large volumes of organic contaminant substances are treated, the following problem may occur. The inflow of water decreases greatly at night and on non-working days. In cases of numerous consecutive non-working days of facility, the good activated sludge microbes are not supplied with a constant amount of contaminant substances, and are reduced in number due to excessive aeration. Further, the sludge breaks up due to the excess load. As a result, stable growth of activated sludge microbes cannot be assured, and bulking or sludge flotation may arise, which tends to reduce the treatment power of the activated sludge. Particularly in cases in which there are large fluctuations in contaminant substance component levels in inflowing waste water for treatment, or in cases in which the load increases sharply due to consecutive non-working days, bulking is prone to occur. To prevent this, larger waste water conditioning tanks may be constructed, storing large quantities of waste water in the hopes of achieving uniform quality of the waste water, which is then continuously supplied at a constant supply rate to the aeration tank; however, problems relating to restrictions of the size of the area of the site and soaring construction costs may be encountered.
(5) The high-speed aeration activated sludge treatment method is a highly efficient waste water treatment method requiring a minimal installation area. However, as the process is a completely mixed system in which the activated sludge and the waste water are constantly present together, the problem of bulking due to filamentous bacteria tends to arise. Moreover, as sewage treatment facilities, such systems are typically classed as secondary treatment facilities, and in recent years facilities that are incapable of adequate treatment have largely disappeared.
In order to solve the above described problems, the inventors previously developed a bioreactor and a waste water treatment method (see Patent Document 1), and achieved good results in waste water treatment in numerous fields.
Moreover, as one method for reducing excess sludge, there has been disclosed a method involving returning excess sludge to the aeration tank after ozone treatment (Non-patent Document 1). Other known methods involve treatment with thermophiles, mechanical crushing, or chemical treatment, followed by return to the aeration tank.
However, as waste water treatments become increasingly diverse, regulations with respect to the environmental load become more stringent, and in some cases it proves difficult to minimize excess sludge, despite performing waste water treatment by the various methods mentioned above.
In particular in the case of food production facilities, contaminant substances in waste water have high oil content. Another problem is that with existing waste water treatment facilities employing conventional methods, the waste water treatment equipment installation sites are quite large in area, making activated sludge treatment insufficient, whereby large amounts of excess sludge arise from inorganic flocculant such as aluminum sulfate using to eliminate oil content are generated.